I have an audio source that only has a signal on the right channel. Sounds fine using my built-in laptop speakers. I then plug in my headphones, the laptop speakers cut out, but I hear nothing in the buds. The channel matrix is set to the default config (i.e., channel 1 to headphones L only, channel 2 to headphones R only). I'm not hearing any sound in the headphones. But, if I change the channel matrix to just channel 2 to headphones L (i.e., channel 1 to headphones L is no longer checked, nor is neither channel checked for headphones R), then I hear sound in both buds. It's behaving as if:
1) Windows 10 has no knowledge of headphones R.
2) Windows 10 (or vMix) is routing headphones L to both buds.
What would explain this behavior?

Note: Mono audio is not enabled in Windows hearing settings. I know if I enable it, then I'll hear sound in both buds without having to fiddle with the channel matrix. But right now, I'm trying to understand the behavior described above.